The present invention relates to the field of ophthalmic lenses, particularly multifocal ophthalmic lenses, and multifocal ophthalmic lenses worn by persons tasking at close quarters to their work, such as computer operators.
The ability of an eye to switch focus from a distant image to a near image depends on the ability of the eye to change its shape. Specifically, certain structures of the eye, such as, for example, the lens, must change its shape or position so that proper focus of light on the retina is achieved. A number of these structures are under muscular control.
The shape of the lens is affected by muscular action. The lens is held in place behind the iris by zonules or suspensory ligaments, which attach to the wall of the eye at the ciliary body. When the ciliary muscles contract, tension on the zonules increases, which allows the lens to increase its curvature and assume a more spheric shape because of its elastic properties.
When light from a distant visual image enters the normal emmetropic eye with a relaxed ciliary muscle, the target is in focus on the retina. However, if the eye is directed at a nearby visual target, the light is initially focused behind the retina, i.e., the image at the retina is blurred, until accommodation occurs. The image is sharpened when the lens becomes thicker with a steeper central curvature because of contraction of the ciliary muscles, resulting in a decreased diameter across the lens as well as its suspensory connections to the wall of the eye via the zonular fibers which become relaxed, allowing the lens to achieve this more spherical shape as needed.
Accommodation refers to the ability of the eye to change its focus. Accommodation is measured by the accommodative amplitude, that is, the power, measured in units called diopters (D), that the lens can vary from the non-accommodative state to a full accommodative state. For example, in accommodation for near vision, the lens increases its curvature, and, as such, the amplitude of accommodation increases.
The lens continues to grow throughout an individual's lifetime. The rate of lens growth is usually about 20 to 30 microns per year. As such, the lens diameter increases over time and this increase has been correlated to a decrease in accommodative facility and thus, a decrease in the ability of the lens to focus on near images. The gradual loss of accommodative power with age means that individual's ability to focus on near images declines over time. When the near point of accommodation has receded beyond a distance comfortable for normal reading, the individual is said to have a condition called presbyopia.
Typically, individuals having presbyopia utilize ophthalmic lenses for correcting their vision. The amount of correction needed in the ophthalmic lenses varies depending upon the distance of the observed object. Commonly, presbyopia leads to individuals utilizing lenses having a double or triple focus (bifocal or trifocal lenses), or lenses wherein the focal distance progressively varies from one point of the lens to another (commonly called progressive lenses).
Even in individuals who have not developed presbyopia and have sufficient accommodative ability, extended periods of viewing at intermediate or near distances can fatigue the accommodative system, and cause symptoms such as eye strain, headaches, blurred vision, neck pain, and general discomfort. It has been found that non-presbyopes can experience relief from these symptoms by wearing lenses containing an appropriate amount of plus power, which in effect moves the individual's resting focus to the distance of the intermediate object, and lessens the need for accommodation to occur in the eyes themselves in order to see clearly. The exact amount of plus power needed to relieve symptoms varies by individual, but typically occurs in the range from +0.50 diopters to +1.50 diopters, over the individual's distance correction.
While presbyopes generally need different amounts of optical power to see clearly at intermediate and near distances, most non-presbyopes can function effectively with the addition of one power for both intermediate and near distances. Thus, when non-presbyopes utilize a lens for relieving the symptoms of extended near and intermediate work, there is little or no need for three separate zones of power in such lenses because two zones of power will suffice (one for distance viewing and one for both intermediate and near distances).
A significant disadvantage to the use of ophthalmic lenses for non-presbyopes, however, is image jump that occurs in bifocal lenses when the view is shifted from the intense near and intermediate viewing of a computer keyboard and screen, for example, to a more distant view. The difficulty is complicated when the individual has no need or little need of a prescription at one of the distinct distance areas of viewing.